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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Carbohydrates, Proteins and Lipids
Macronutrients are the nutrients we need in large amounts to provide energy and support growth. The three main macronutrients are carbohydrates, proteins and lipids (fats). Each plays a vital role in keeping our bodies functioning properly.
Key Definitions:
Carbohydrates are our body's main source of energy. They're made of carbon, hydrogen and oxygen atoms, usually in a ratio of 1:2:1 (hence "carbo-hydrates").
Monosaccharides: Single sugar units like glucose, fructose and galactose. Glucose is the body's preferred energy source.
Disaccharides: Two monosaccharides joined together, such as:
Polysaccharides: Many sugar units joined together.
Carbohydrates have the general formula (CH2O)n, where n is the number of carbon atoms. Glucose, the most important monosaccharide, has the formula C6H12O6.
When we eat starchy foods, an enzyme called amylase in our saliva begins breaking down the starch into maltose. That's why if you chew a piece of bread for long enough, it starts to taste sweet!
Proteins are complex molecules made up of amino acids. They're essential for growth, repair and regulation of the body's tissues and organs.
Amino acids are the building blocks of proteins. Each amino acid has the same basic structure:
There are 20 different amino acids used to build proteins in the human body. Of these, 9 are considered "essential" because our bodies cannot make them – we must get them from our diet.
Proteins have four levels of structure:
Proteins have many vital roles:
Kwashiorkor is a severe form of protein deficiency often seen in developing countries. Symptoms include a swollen belly, changes in skin and hair colour and stunted growth. This highlights the critical importance of proteins in our diet, especially for growing children.
Lipids are a diverse group of organic compounds that are insoluble in water but soluble in organic solvents. The main types of lipids in our diet are triglycerides (fats and oils), phospholipids and steroids.
Triglycerides are the most common dietary lipids. They're made of:
Saturated fatty acids: No double bonds between carbon atoms. Usually solid at room temperature (e.g., butter, lard).
Unsaturated fatty acids: Have one or more double bonds between carbon atoms.
Cholesterol is a type of lipid that's essential for our bodies. It's used to make cell membranes, hormones and vitamin D. However, high levels of blood cholesterol can lead to heart disease.
These are essential fatty acids that our bodies cannot make. Omega-3s are found in oily fish like salmon and mackerel, while omega-6s are found in vegetable oils. They're important for brain function and reducing inflammation.
A balanced diet should include all three macronutrients in the right proportions:
Should make up about 50-55% of your daily energy intake.
Focus on complex carbohydrates and fibre from whole grains, fruits and vegetables.
Should make up about 15-20% of your daily energy intake.
Good sources include meat, fish, eggs, dairy, beans and nuts.
Should make up about 30% of your daily energy intake.
Focus on unsaturated fats from plant oils, nuts and fish rather than saturated fats.
Imbalances in macronutrient intake can lead to health problems:
The UK government's Eatwell Guide recommends that a balanced diet should include:
Carbohydrates, proteins and lipids are the three main macronutrients essential for human nutrition:
A balanced diet containing all three macronutrients in the right proportions is essential for good health and wellbeing.
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